As you start to build your woodworking skills I recommend starting with the circular saw. It is the first saw you should buy and will take you a long way toward learning basic woodworking. It does has some limitations such as not able to cut an inside corner or rip narrow boards, but for general straight line cutting it is possibly the best power tool available.
When you go to buy look for something in the $75-$100 range. If you are building houses for a living you will want something at a higher price point, but if you use a saw that much you wont be reading this post. When you are picking out a saw check that the blade doesn’t have side-to-side play and I prefer to have a laser on it as it helps free-hand cuts. The adjustment you use the most is the depth of the blade, so make sure that it is something you find easy to use.
The standard size saw and one you should buy is 7 1/4″, which is the blade size. The saw will likely come with a blade that will work fine cutting 2×4 type of lumber for rough construction, but plan on buying a 40 tooth blade for about $20 to go with it. This blade will let you make fairly smooth cuts in both lumber and plywood.
If you are planing on building plywood projects you should also consider some kind of fence. A fence is something that guides the saw and helps you make straight cuts. When breaking down 4′ x 8′ sheet goods you need to make a lot of long straight cuts and a fence really helps. You can buy a fence that is usually an aluminium track that is about 9′ long and can be split into two pieces so you can cut either across the 4′ or 8′ length. You can also make your own. A speed square, drywall square and chalk line are also helpful in laying out your cuts.